No evidence of paternal genetic contribution to nestling cell-mediated immunity in the North American barn swallow

Females involved in extrapair mating are generally thought to obtain indirect benefits through increased offspring quality. We investigated this in the barn swallow, Hirundo rustica, a socially monogamous passerine in which females seem to have a preference for elaborately ornamented, high-quality males, both as social and extrapair mating partners. In this species, a variety of parasites and pathogens can potentially infect individuals, both during the nestling stage and later in life. The ability to cope with infections and hence the quality of the immune system may therefore have immediate survival or future fitness consequences. To evaluate whether females engage in extrapair mating to increase offspring cell-mediated immunity, a pivotal component of the avian immune system, we experimentally challenged nestling immunity. Body condition, rearing environment and gender affected the ability of nestlings to mount an immune response. Contrary to a prediction derived from the ‘genetic quality’ hypothesis, our results revealed no enhanced cell-mediated immune responsiveness in extrapair young compared to their maternal half-siblings reared in the same brood. Female barn swallows therefore do not seem to mate with multiple males in order to increase nestling cell-mediated immunity. Although we cannot reject the possibility that extrapair mating is beneficial to female barn swallows, our results offer no support to the hypothesis that females obtain genetic benefits through extrapair fertilizations